Well, we are pretty close, honestly.
Right now, with one bog-standard android tablet I can have a Kindle app, a Nook app, a Kobo app, and Libby. Functionally, I pay different people, but it is one device for all my reading needs. That's agnostic enough.
This falls apart for e-ink screen devices, which I prefer for reading, that cannot currently handle all those sources on one device. I can get a Kobo that handles Kobo and a little bit of Libby functionality, but won't talk to Nook or Kindle.
I may end up rooting my older tablet, downloading by nook library, and then using Kobo's google drive/dropbox functionality to get my library moved over....